1. Field of the Invention
This invention relates to an optical scanner driving apparatus and an optical scanner driving method of driving an optical scanner which scans a light from a light source one-dimensionally or two-dimensionally.
2. Description of the Related Art
There are generally known optical scanners that are prepared based on semiconductor manufacturing technologies for scanning a light from a light source one-dimensionally or two-dimensionally (see U.S. Pat. No. 5,606,447 and U.S. Pat. No. 6,188,504). These scanners are characterized by compactness and low profile.
FIG. 1 illustrates a principle of an optical scanner. Referring to FIG. 1, a moving plate 1 having a side to be used as a mirror has a thin rectangularly parallelepipedic profile. A pair of spring sections (elastic members) 2 made of metal or a semiconductor material are arranged respectively at middle positions of the longitudinal edges thereof. A coil pattern (to be referred to as a “driving coil” hereinafter) 3 is arranged on the back side of the moving plate 1. A pair of permanent magnets 4 is arranged opposing to the respective lateral edges of the moving plate 1. The permanent magnets 4 generate a magnetic field having a component running in a direction (B) perpendicular to the wiring section 3a of the driving coil 3 that is parallel with the lateral edges of the moving plate 1.
As an AC current having a driving frequency f flows through the driving coil 3 of an optical scanner having the configuration as described above, a magnetic field of the permanent magnets 4 generates a force according to the Fleming's left hand rule in a direction perpendicular to the major surfaces of the moving plate 1. The magnetic field of the permanent magnets 4 is generated perpendicular to the direction of the electric current flowing in the wiring section 3a. Then, the moving plate 1 vibrates around the spring sections 2 operating as rotary axis with a frequency of f due to the generated force and the resilient force of the spring sections 2. If the AC current I is expressed by I=I0 sin(2πft), the intensity of the magnetic field is H (magnetic flux density B), the number of turns of the driving coil 3 is N, the area of the driving coil 3 is S and the magnetic permeability in vacuum is μ0, the scan angle θ and the generated torque F are defined by the equation (1) below:F=μ0NHSI0 sin(2πft)·cos θ  (1)
In the equation (1), the scan angle θ can be determined by solving the equation of motion (2) below:J{umlaut over (θ)}=−kθ−C{dot over (θ)}+F  (2)where k is the spring constant of the spring sections 2, C is the damping coefficient and J is the moment of inertia of the optical scanner. If the mechanical resonance frequency of the optical scanner is fc, k is expressed by equation k=J·(2π·fc)2.
Meanwhile, if the scan angle θ is small and cos θ≈1 can be assumed, the relationship between the scan angle θ and the driving frequency f of the AC current I is expressed by equation (3) below by using the above equations (1) and (2):
                              θ          ⁡                      (            f            )                          =                                                            μ                0                            ⁢                              NHSI                0                                      J                    ⁢                                    1                                                                    {                                          k                      -                                                                        (                                                      2                            ⁢                            π                            ⁢                                                                                                                  ⁢                            f                                                    )                                                2                                                              }                                    2                                +                                                      B                    2                                    ·                                                            (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        f                                            )                                        2                                                                                                          (        3        )            
FIG. 2A shows the frequency response characteristics relative to the scan angle θ determined by the above equation (3). FIG. 2B shows the frequency response characteristics relative to a phase difference between the scan angle θ and the driving signal. In FIG. 2A, the vertical axis indicates the scan angle θ and the horizontal axis indicates the driving frequency f. In FIG. 2B, on the other hand, the vertical axis indicates the phase difference θ and the horizontal axis indicates the driving frequency f. From FIG. 2A, it can be seen that a large scan angle θ (resonance amplitude) can be obtained by making the driving frequency f correspond to the mechanical resonance frequency fc. Therefore, it is a common practice to make the driving frequency f of the AC current I and the mechanical resonance frequency fc correspond to each other when driving the moving plate 1. Note that, as shown in FIG. 3B, the phase of the scan angle θ (equivalent to the vibration of the moving plate 1) of the moving plate 1 delays by 90° relative to that of the driving signal (or the AC current I) shown in FIG. 3A.
The state of vibration of the moving plate 1 needs to be monitored constantly in order to stably drive the optical scanner. Therefore, the moving plate 1 is provided with a sensor for detecting the state of vibration of the scanner. Such a sensor is disclosed, for example, in U.S. Pat. No. 6,188,504. The sensor disclosed in this patent document has a configuration as shown in FIG. 4. On the surface of the moving plate 11′, a coil pattern (to be referred to as a “sensing coil” hereinafter) 5 that differs from the driving coil 3 is arranged. An electromotive force is generated by linking the magnetic field of the permanent magnets 4 with the sensing coil 5 when the moving plate 11′vibrates. The electromotive force Vr generated in the sensing coil 5 is defined by the equation (4) below:Vr=NSBSS·dθ/dt·cos θ  (4),where NS is the number of turns of the sensing coil 5, B is the magnetic flux density and SS is the area of the sensing coil 5.
If the driving signal (i.e., the AC current I) applied to the optical scanner is I=I0 sin(2πfC·t) in the above arrangement, a phase of the vibration of the optical scanner delays by 90° for the driving signal. Therefore, the above equation can be replaced by θ=−θ0 cos(2πfC·t). Then, if the scan angle θ (θ0) is small, the electromotive force expressed by the equation (4) can be approximated by the equation (5) below.Vr=NsBSsθ02πfc sin(2πfct)cos{−θ0 cos(2πfct)}≈NsBSsθ02πfc sin(2πfct)  (5)Therefore, as shown in FIG. 3C, the phase of the electromotive force (or the sensing signal) generated in the sensing coil 5 advances by 90° with reference to that of the vibration of the moving plate 11′ shown in FIG. 3B. Note that the sign of the electromotive force is inverted and the phase of the electromotive force delays by 90° when the connections of the opposite ends of the sensing coil 5 are switched. In this specification, however, it is assumed that the phase of the electromotive force advances by 90°. The phase difference is always 90° regardless of the driving frequency. Thus, driving the optical scanner based on the resonance frequency provides phase relationships as illustrated in FIGS. 3A through 3C among the driving signal, the vibration of the moving plate 11′, and the electromotive force (sensing signal) of the sensing coil. The phase of the driving signal corresponds to that of the sensing signal.
FIG. 5 shows an example of the driving apparatus as described above. A moving plate 11 has a driving coil 11a and a sensing coil 11b. When an operation controller such as a personal computer (not shown) supplies a control circuit 12 with a control signal indicating specification values for the vibration amplitude (scan angle) and the vibration frequency of the moving plate 11, the control circuit 12 outputs a driving reference signal to the driving circuit 13 according to the control signal. The driving circuit 13 outputs a driving signal to the driving coil 11a according to the driving reference signal. As a result, the moving plate 11 vibrates with a predetermined scan angle and a predetermined vibration frequency. At this time, an electromotive force (sensing signal) is generated at both ends of the sensing coil 11b by the electromagnetic induction caused by the linkage between the sensing coil 11b and the magnetic field. From the equation (5), it is possible to assume that the sensing signal has an amplitude proportional to the vibration frequency and the scan angle of the moving plate 11 and forms a sinusoidal wave having the same frequency as the vibration frequency. The sensing signal is transmitted to the control circuit 12 by way of a detection circuit 14. The control circuit 12 monitors the sensing signal and corrects the driving reference signal output to the driving circuit 13 when the vibration amplitude (scan angle) and the vibration frequency of the moving plate 11 deviate from respective predetermined values. In this way, the moving plate 11 is controlled based on the sensing signal.
The above-mentioned optical scanner driving apparatus requires the sensing coil 11b to be arranged along with the driving coil 11a on the same surface of the moving plate 11. Then, the area and the number of turns of the driving coil 11a are limited, reducing the drive efficiency of the optical scanner. While this problem may be avoided by using the large moving plate 11, the large moving plate 11 reduces the resonance frequency. Then, the scope of application of such an optical scanner will become limited. Further, the manufacturing process will become complicated, reducing the reliability and increasing the manufacturing cost. A sensor other than the sensing coil may be introduced. However, such a sensor may be costly and necessitate a cumbersome operation of regulating the alignment with the optical scanner.
To solve the above identified problem, for example, there is disclosed a driving circuit to detect an angular velocity zero moment of the vibration mirror and start an oscillation pulse (Japanese Patent Application KOKAI Publication No. 10-207973). However, this method cannot continuously control the vibration amplitude and the vibration frequency. The high-precision control is unavailable.